There is a need for high specific energy batteries that deliver more power per unit battery mass in order to power devices that are increasingly sophisticated that require correspondingly higher power requirement. The higher the specific energy, the less battery mass required to power a given device. To meet the demand for high specific energy batteries, we disclose a novel high voltage, high specific capacity Li-ion battery based on dual intercalation of a Li salt into graphite electrodes. In particular, these batteries, in principle, can reach a factor of 3.8 greater specific energy over state-of-the-art Li-ion batteries based on higher cell voltage and higher specific capacity.
State-of-the-art lithium electrochemical cells (e.g., U.S. Pat. Nos. 6,852,446, 6,306,540, 6,489,055) generally employ a graphitic carbon anode (e.g. mesocarbon microbead carbon), a lithiated transition metal oxide cathode (e.g., LiCoO2), and a highly conductive electrolyte solution to provide mobility to Li ions, which are transported from the anode to the cathode during discharge, and vice versa during charge. The salt used in the electrolyte affects cell performance and should be highly conductive, have high thermal stability, be electrochemically stable above the potential of the fully charged cell, and be nontoxic.
Electrolyte fluids in current state of the art lithium ion batteries generally consist of a solvent for ionizable salt, wherein the ionizable salt is, for example, LiClO4, LiBF4, LiAsF6, LiSbF6, and LiPF6. The most common salt in use is LiPF6, which is added to organic carbonate solvent mixtures to form the electrolyte solution. The size the anion in each of these salts is relatively large, and can be larger (e.g., about 0.45 nm for PF6−) than the spacing between the graphene planes in a graphite electrode (0.35 nm). Accordingly, insertion (and deinsertion) of these relatively large anions in a graphite electrode impart stresses to the graphite. Because rechargeable batteries charge and discharge in multiple cycles, these repeated insertion and deinsertion stresses can quickly degrade the graphite leading to graphite exfoliation and loss of electrode function.
There has been some investigation of batteries having a pair of carbonaceous electrodes (U.S. Pat. Nos. 4,865,931, 4,830,938). However, cells having a pair of intercalating electrodes and an anion provided by the electrolyte suffer from significant capacity loss after several cycles, and cannot be (dis)charged at high rates. Capacity fade and low rate capability for other dual intercalating cell designs are not surprising because of the stresses imparted by insertion and deinsertion of the relatively large diameter polyatomic anions (e.g., PF6− having diameter of 0.45 nm) into the graphite host (unstressed graphene planes separated by 0.35 nm). These stresses are associated with “destructive intercalation” (U.S. Pat. No. 5,532,083) and result in exfoliation of the graphite and attendant degradation of the electrode. Accordingly, there is a need in the art for electrochemical cells and batteries capable of utilizing smaller anions that do not stress intercalating electrodes.
The choice of a suitable anion, however, is constrained by a number of criteria: (i) It must be electrochemically stable over a wide voltage window; (ii) It must be small enough to insert (intercalate) and deinsert (deintercalate) into the graphene sheets; (iii) It must be soluble in non-aqueous organic solvents; (iv) It must yield solutions with acceptable conductivities. Although such systems have been studied (U.S. Pat. No. 6,022,643), no characterization of Li-ion cells with carbonaceous cathodes and small diameter anions have been reported.
LiF meets the first two criteria, with the anion being stable over a wide voltage window and being small. However, LiF is insoluble in virtually all organic solvents, which seemingly prevents its use for the dual intercalating cell design. In addition, the high potential of the Li—F redox couple negates the use of standard Li-ion battery solvents that can decompose above roughly 4.3V versus Li+/Li. Therefore, to avoid sub-optimal battery performance, the nonaqueous solvent must be carefully chosen.
Recently, synthesis of fluorinated boron-based anion receptors (“AR”) for nonaqueous solutions has been reported. U.S. Pat. Nos. 6,022,643, 6,120,941, 6,352,798. These AR enhance the ionic disassociation of a variety of lithium salts in low dielectric solvents, by incorporating non-hydrogen bonded electrophilic groups that are stable over a wide electrochemical stability window of approximately 5V. AR, when dissolved in conventional Li-ion battery electrolyte solvents such as ethers and aliphatic carbonates, enhance the dissolution of lithium salts, including lithium halides, resulting in solubility increases by upwards of six orders of magnitude. The conductivity of these electrolyte solutions is similar to conventional Li-ion electrolyte solutions (e.g., 3.58×10−3 S/cm for 1 M LiF in 1:2 EC-DMC for 1 M borate AR at 25° C.). It is important to note that the studies examining AR restrict AR use to electrolyte conductivity measurements, and have not examined their use for dual ion intercalating batteries. Whether these AR can be used in a cathode-intercalating battery system is unclear for a number of reasons. For example, the AR may hinder anion intercalation at the cathode. In addition, there will be a potential drop at the cathode associated with de-complexing the anion from the AR, manifesting as a polarization loss, which will lower the operating voltage during operation.
Another challenge to overcome is salt starvation as the cell is charged. During charge, the electrolyte is deprived of salt which decreases electrolyte conductivity. LiF is a useful salt because it is of low molecular weight so that supersaturating the electrolyte with LiF will not dramatically impact the overall cell gravimetric energy density. In addition, use of the AR further minimizes any adverse impact of high concentration LiF.
The rechargeable devices of the present invention that use LiF dissolved in nonaqueous, high voltage stability organic solvents with an anion receptor additive, are attractive for a number of reasons besides providing higher specific energies. The systems are safer because discharged cathodes are graphite, and charged cathodes (fluorinated graphite) do not thermally decompose until 400° C. The systems are environmentally friendlier in that graphite cathodes are not toxic and/or carcinogenic compared to the lithiated transition metal oxide cathodes used in state-of-the-art Li-ion batteries. The batteries of the present invention are also generally lower in cost with respect to cathode composition as graphite cathodes are inexpensive compared to lithiated transition metal cathodes.